1. Field
The present disclosure relates to light emitting devices, and more particularly, to light emitting devices with improved electrode structures. Various configurations of electrode structures together with a relatively thin transparent conductive oxide layer may increase light extraction and lower operation voltage for a light-emitting device.
2. Background
Light-emitting diodes (LEDs), which convert electric current to light, are one of the most important solid-state light-emitting devices nowadays. An LED generally includes a light-emitting layer located between a P-type semiconductor layer and an N-type semiconductor layer. The driving current is applied to a P-type electrical contact, which is electrically connected to the P-type semiconductor layer, and to an N-type electrical contact, which is electrically connected to the N-type semiconductor layer. The P-type semiconductor layer and the N-type semiconductor layer thus eject holes and electrons respectively to the light-emitting layer. The holes and electrons combine within the light-emitting layer and emit light accordingly. The light is emitted in all directions from the light-emitting layer and then leaves the surfaces of the LED.
Increasing the size and the light-emitting area of an LED is one of the methods to enhance its luminous efficiency and luminous intensity. For a conventional nitride-based LED, however, increasing its size may be subject to limitations since electric current cannot be spread uniformly from the electrical contact across the entire light-emitting layer. For instance, because a P-type nitride-based semiconductor layer has relatively low conductivity, the spread of electric current applied to the P-type electrical contact may be limited to a certain area of the P-type nitride-based semiconductor layer, below the P-type electrical contact. The current cannot be spread laterally across the entire P-type nitride-based semiconductor layer. Moreover, heat may be generated at certain parts of the LED, causing the material of the components around the electrical contact to deteriorate more quickly. As for the N-type nitride-based semiconductor layer, though it has better conductivity, there is still some resistance for the current to spread laterally across the layer. As the size of the LED device increases, electric current cannot be spread as uniformly from the electrical contact across the N-type nitride-based semiconductor layer. Therefore, the size of a conventional nitride-based LED is limited by the lateral spread of electric current in the P-type nitride-based semiconductor layer and in the N-type nitride-based semiconductor layer.